A document camera is a special video camera designed to make a presentation, hold a meeting or give a lecture in classroom, boardroom, or conference. In the document camera, lens and some adjustable mechanical structures cooperate with each other to capture the image of a two or three-dimensional object, and the captured digital image of the object is shown on a display device to be viewed by the user and the audience. Moreover, through a data line, the captured digital image of the object may be stored in a storage device or shared with other users.
FIG. 1 is a schematic diagram illustrating a document camera for capturing and showing an image of an object according to the prior art. As shown in FIG. 1, the document camera 10 has a lens 101 for focusing and capturing the image of an object 11. The object 11 is for example a fish-like sculpture. After the fish-like sculpture 11 is placed under the lens 101, the fish-like sculpture 11 is shot by the lens 101, and the captured digital image 121 is shown on a display device 12.
When the digital image 121 of the object 11 is shown on the display device 12, the aspect ratio of the object 11 may be realized. For example, the size of the fish body relative to the fish tail can be known from the digital image 121. However, the actual size of the fish-like sculpture 11 fails to be realized from the digital image 121. That is, the digital image 121 shown on the display device 12 is the magnified or minified image of the object 11, but the actual size of the object 11 fails to be directly realized by referring to the digital image 121.
For facilitating the user to realize the actual size of the object 11, a coin (not shown) or a ruler 13 may be placed beside the object 11. By comparing the image of the ruler 13 with the digital image 121, the user may estimate the actual size of the object 11. Another approach provides graduations on the lens 101. By referring to the graduations, the user may estimate the actual size of the object 11.
Although the ruler 13 or the graduations may facilitate the user to estimate the size of the object 11 according to the digital image 121 shown on the display device 12, there are still some drawbacks. For example, the ruler 13 should be additionally prepared and the estimated size of the object 11 is obtained through size conversion in one's head, but not directly obtained. In addition, if the object 11 needs to be periodically and frequently shot, the object 11 and the ruler 13 should be adjusted for each shot. As such, it is inconvenient to operate the document camera 10. For example, it is troublesome for the user to record and observe the plant growth every hour.
Moreover, since the actual size of the object 11 is obtained through size conversion, the resolution or the orientation of the lens 101 may incur measuring error regardless of whether the ruler 13 is placed at the lens side or the object side. After the digital image 121 is acquired, the position and orientation of the ruler image in the digital image 121 is fixed. For example, when the user wants to realize the diagonal length of the object 11, but the ruler 13 is horizontally placed, the diagonal length is usually obtained by inaccurate estimation.
Moreover, since the size conversion about the actual size is interpreted in an inaccurate manner, it is difficult to estimate the length of a curve or the area of an irregular pattern.
From the above description, the conventional document camera is not feasible to estimate the actual size of an object. Therefore, there is a need of providing an improved document camera so as to obviate the drawbacks encountered from the prior art.